I love the idea of a Far Realm adventure path. There are plenty of "theme" monsters to use for it, including a few Epic ones that very rarely see the light of (a real?) day otherwise.
Given that the first three adventure paths all had BBEGs with CRs above 20, and in fact AoW had an actual god for the BBEG, setting up an Uvuudaum as the "end boss" for the Far Realm AP is a gimme. CR 27 is perfect for an end-boss in this sort of situation, and I can attest from personal experience that an Uvuudaum when properly played (and hyped up before its big reveal) is nasty.
Possibly it could come with a Neh-Thalggu minion or two, whether or not that means the "juniors" with only 6 legs, or the "adult" in the ELH that has 12. Along the way, feature appearances by critters like Kaorti, Tsochar, Mind Flayers and Aboleths (of course), Mooncalves (come on, we all know they look more like Cthulhu than cows), and numerous sundry pseudonatural beasts.
If WotC wants to delve back into the world of OGC, there are several other templates out there that fit extremely well, such as Non-Euclidean, and monsters such as the Lumina from Legends of Avadnu. Unlikely, I know, but there are good resources out there.